Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of super alloy gas turbine engine components.
Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In gas turbine engine applications, efficiency is a prime objective.
Improved gas turbine engine efficiency can be obtained by operating at higher temperature, however current operating temperatures in the turbine section exceed the melting points of the super alloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections.
The ceramic cores themselves may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into dies. After removal from the dies, the green cores are thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile.
One known process includes forming one or more refractory metal cores (RMCs) that includes a combination of cutting (e.g., laser cutting or stamping) from a refractory metal sheet (e.g., molybdenum or niobium), forming/shaping (e.g., the stamping or other bending), and coating with a protective coating. The RMC(s) are then transferred to a die where a ceramic material (e.g., silica-, zircon-, or alumina-based) is formed over a portion of the RMC(s) to form an initial combination (core assembly). The as-molded ceramic material may include a binder. The binder may function to maintain integrity of the molded ceramic material in an unfired green state. The combination may be then transferred to a heating chamber (e.g., kiln or furnace) for further processing.
Conventional ceramic cores are produced by a molding process using ceramic slurry and a shaped die; both injection molding and transfer-molding techniques may be employed. The pattern material is most commonly wax although plastics, low melting-point metals, and organic compounds such as urea, have also been employed. The shell mold is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia and alumina silicates.
The investment casting process to produce a turbine blade includes using a ceramic core having geometry desired for the internal cooling passages for the blade is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core, is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point the mold is thoroughly dried and heated to an elevated temperature to remove the wax material and strengthen the ceramic material.
The result is a ceramic mold that contains a ceramic core that defines a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the super alloy casting to be made. The core and shell may also define casting portions such as gates and risers which are necessary for the casting process but are not a part of the finished cast component.
After the removal of the wax, molten super alloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are then removed from the super alloy casting by a combination of mechanical and chemical means.
As previously noted, the currently used ceramic cores limit casting designs because of their fragility and because cores with dimensions of less than about 0.012-0.015 inches (0.305-0.381 mm) cannot currently be produced with acceptable casting yields.